A mysterious group of ancient animals may have been the ancestors of starfish, according to a study of newly discovered fossils.
鈥淚t鈥檚 the first time that soft parts have been found in this group of fossils,鈥� says聽聽of Claude Bernard University Lyon 1 in France. 鈥淚t was the only clue to put an end to a very old debate.鈥�
Stylophorans are a group of early complex animals. They appeared in the Cambrian period, which began 541 million years and saw the first explosive flowering of animal diversity, and disappeared during the Carboniferous, which ended 299 million years ago.
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Each fossil looks a little like an arrow, if the arrowhead was disproportionately large compared to the shaft. They do not fit neatly into any existing group of animals, so the question has been聽.
Three competing ideas
One interpretation is that the 鈥渟haft鈥� is a tail, in which case they looked a bit like tadpoles. That would suggest they were closely related to backboned animals like fish and humans. This idea has been聽聽by the British palaeontologist Richard Jefferies.
However, the fossils also look a bit like聽echinoderms, the group that includes聽starfish. In that interpretation, the shaft is not a tail but a feeding arm. The difficulty with this is that stylophorans only had one arm, whereas starfish聽typically聽have five.
Some have also argued that they were hemichordates, a group of worm-like creatures closely related to echinoderms.
Although they look very different today, echinoderms, backboned animals and hemichordates all belong to a聽big group called the deuterostomes, and would have looked relatively similar early in their evolution. If stylophorans are representatives of that early evolution they could fit into any one of the three groups 鈥� but no one could agree where they belong.
鈥淚t wasn鈥檛 obvious at all,鈥� says Lefebvre.
Prototype starfish
He and his colleagues might be about to settle the debate. They have unearthed thousands of new stylophoran fossils at a site in Morocco. After a decade of searching, they found some with soft tissues preserved.
This revealed that stylophorans had additional echinoderm-like traits that had not been seen before. 鈥淲e show they don鈥檛 have a head and tail, they have a body and an arm,鈥� says Lefebvre.
Crucially, stylophorans had a 鈥渨ater vascular system鈥�, which is the echinoderm equivalent of the human circulatory system. 鈥淲e have blood, echinoderms have seawater,鈥� says Lefebvre. 鈥淭hey don鈥檛 have veins, but tubes in which [seawater] circulates in the body.鈥�
A water vascular system is an unambiguous sign that stylophorans are echinoderms, says Lefebvre. 鈥淚t鈥檚 unique,鈥� he says. Even hemichordates, echinoderms鈥� closest living relatives, don鈥檛 have it.
The finding will force a rethink about the early evolution of animals, Lefebvre says.
He says stylophorans have often been held up as an example of very early deuterostomes, but they are too far evolved as echinoderms for that to be true. 鈥淭hese animals should not be considered for discussion about deuterostome origins,鈥� he says.
That leaves the evolutionary events that occurred early in our deuterostome evolution something of a mystery.
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